some hubs are easier to do than others. I think Surly hubs can be done in about 10 minutes, since they use nice standard threadings. Just pull out the solid axle, and put in a hollow one. Other brands, it's the same idea, but comparability might be an issue. As long as you can find an axle to fit the cones you should be golden. You might need to cut it down to size, I don't think any QR axles come stock in 120mm